This invention relates to solvent extraction. It also relates to a method for liquefying coal using a selective solvent. It particularly relates to a process for obtaining valuable liquid hydrocarbons from particulate coal utilizing the steps of solvation and hydrogenation.
It has long been known that hydrocarbon gases, liquids, pitch and chemicals may be obtained in useful form from coal which is mined from the earth. Usually, the prior art has employed destructive distillation or other gasification processes for the conversion of coal into these more valuable and useful products. Recently, the prior art has developed a high pressure hydrogenation of coal technique to effectuate such conversion. Still more recently, methods involving solvent extraction techniques have been developed for obtaining useful fuels and chemicals from coal whereby the coal is contacted with a selective solvent which acts as a hydrogen-donor for supplying sufficient hydrogen to the coal to aid in converting it into a liquid state. Following the solvent extraction step the prior art schemes have utilized various recovery procedures, such as hydrogenation of the liquid coal extract, for increasing its value and utility together with retorting or coking of the residual materials obtained from the solvent extraction step to still further convert these coal derived products into more commercially valuable products.
Furthermore, the prior art has taught the use of various catalysts to enhance the recovery of hydrocarbons from coal liquefaction. In previous procedures, there has been a remaining persistent problem and that is the problem of recovering the catalyst from the hydrocarbon product and the residual ash. Because of the cost of catalysts, it has been thought that for a coal liquefaction process to be economical, the catalyst must be recovered and subsequently recycled. An alternative would be to utilize a once-through or throw-away catalyst, which would only have to be separated from the hydrocarbon product and could be discarded with the residual ash.
Therefore, for these and other reasons, none of the aforementioned prior art procedures have been sufficiently commercially attractive or feasible to warrant widespread commercial exploitation of converting coal into valuable liquid products. Generally, the deficiencies in the prior art schemes have not only involved capital investment problems and lack of suitable and economical catalyst systems, but have also involved liquid product quality and quantity problems which have yet to be solved in an economical and facile manner.
Since it is clear to those skilled in the art that the vast mineral resources of bituminous coal represent an extremely important supply of energy and an extremely important source of raw materials for valuable chemicals, it would be desirable to improve upon the prior art techniques, particularly the solvent extraction step in order to reduce the cost of obtaining high quality petroleum-type product from coal.